[PATCH kmod] libkmod: avoid undefined behaviour in libkmod-builtin.c:get_string

[PATCH kmod] libkmod: avoid undefined behaviour in libkmod-builtin.c:get_string

[Eugene Syromiatnikov]

Static analysis has reported a potential UB:

    kmod-31/libkmod/libkmod-builtin.c:125: use_invalid: Using "nullp", which points to an out-of-scope variable "buf".
    #  123|   	size_t linesz = 0;
    #  124|
    #  125|-> 	while (!nullp) {
    #  126|   		char buf[BUFSIZ];
    #  127|   		ssize_t sz;

It seems to be indeed an UB, as nullp is getting assined an address
inside object buf, which has a lifetime of the while loop body,
and is not available outside of it (specifically, in the while
condition, where nullp is checked for NULL).  Fix it by putting
buf definition in the outer block.
---
 libkmod/libkmod-builtin.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libkmod/libkmod-builtin.c b/libkmod/libkmod-builtin.c
index fd0f54923a48..40a7d6142d03 100644
--- a/libkmod/libkmod-builtin.c
+++ b/libkmod/libkmod-builtin.c
@@ -105,11 +105,11 @@ static off_t get_string(struct kmod_builtin_iter *iter, off_t offset,
 			char **line, size_t *size)
 {
 	int sv_errno;
+	char buf[BUFSIZ];
 	char *nullp = NULL;
 	size_t linesz = 0;
 
 	while (!nullp) {
-		char buf[BUFSIZ];
 		ssize_t sz;
 		size_t partsz;
 
-- 
2.28.0

Re: [PATCH kmod] libkmod: avoid undefined behaviour in libkmod-builtin.c:get_string

[Lucas De Marchi]

On Tue, 13 Aug 2024 16:17:27 +0200, Eugene Syromiatnikov wrote:
> Static analysis has reported a potential UB:
> 
>     kmod-31/libkmod/libkmod-builtin.c:125: use_invalid: Using "nullp", which points to an out-of-scope variable "buf".
>     #  123|   	size_t linesz = 0;
>     #  124|
>     #  125|-> 	while (!nullp) {
>     #  126|   		char buf[BUFSIZ];
>     #  127|   		ssize_t sz;
> 
> [...]

Applied, thanks!

[1/1] libkmod: avoid undefined behaviour in libkmod-builtin.c:get_string
      commit: 5c22362b6b97af9c6b7587f0c3450001e9893115

Best regards,
-- 
Lucas De Marchi <lucas.de.marchi@gmail.com>